Moon phase calculator: phase name, illumination and age for any date
Enter a date and the calculator returns the Moon's phase name, illuminated fraction, age in days since the last new moon, and the dates and times of the next full and new moon. Everything is computed locally from the same engine that powers our monthly sky guides — no request leaves your browser. It is the fastest way to answer the two questions every observer has on a clear evening: how much Moon will be in my sky tonight, and when does the next truly dark window open.
What the numbers mean
Illumination is the fraction of the lunar disc lit by the Sun as seen from Earth, from 0% at new moon to 100% at full. It grows fastest near the quarter phases and slowest near new and full — which is why a crescent seems to thicken visibly night after night while a gibbous Moon hardly changes.
Moon age counts days since the exact instant of the previous new moon. The mean cycle is 29.53 days, but the true interval swings between about 29.27 and 29.83 days across the year because the lunar orbit is elliptical. This engine locates the exact previous new-moon instant rather than assuming an average month, so the age stays honest even for extreme months.
Phase names — waxing crescent, first quarter, waxing gibbous and their waning twins — are conventions applied between the four principal instants. We use 45-degree elongation bands centred on new, first quarter, full and last quarter. Other tools may slice the cycle differently (some label the entire quarter-span "first quarter"), so border-day labels can legitimately differ between apps. If you want the full story of traditional names like Harvest or Strawberry Moon, read moon phase names, explained.
Methodology
Phase instants are computed with the algorithm from Jean Meeus's Astronomical Algorithms
(2nd edition), chapter 49: a mean phase from the J2000 new-moon epoch (JDE 2451550.09766) and the mean
synodic month (29.530588861 days), corrected by the chapter's periodic terms for the Moon's and Sun's
anomalies, lunar latitude and planetary perturbations, then converted from Terrestrial Time to UT with
the Espenak–Meeus (2006) ΔT polynomial. Illuminated fraction is derived from the geocetric elongation
between the Sun and Moon using the NOAA solar formulas and the Astronomical Almanac's abridged lunar
series. All formulas are published, public-domain mathematics — the module is a dependency-free
pure-function implementation you can audit in
assets/js/moon.js.
Validation
Expected values below are official published numbers, fetched on 2026-11-16 from
the US Naval Observatory's Dates of Primary Phases of the Moon service
(2026 /
2027, times in UT),
cross-checked against timeanddate.com's UTC table
(agreement within 1 min). All 25 primary phases from October 2026 through March 2027 matched
the USNO table with |Δ| ≤ 2 minutes (tolerance ±5 min); the full table ships in this site's
README and tests.mjs. A sample:
| Phase | Date | USNO (UT) | This engine (UT) | Δ |
|---|---|---|---|---|
| New Moon | 2026-11-09 | 07:02 | 07:02 | 0 min |
| Full Moon | 2026-12-24 | 01:28 | 01:28 | 0 min |
| First Quarter | 2026-12-17 | 05:42 | 05:44 | +2 min |
| Last Quarter | 2027-01-29 | 10:55 | 10:54 | −1 min |
| Full Moon | 2027-02-20 | 23:23 | 23:23 | 0 min |
| New Moon | 2027-03-08 | 09:29 | 09:29 | 0 min |
Engine disclosure
Engine: Meeus public-domain algorithm (ch. 49, full periodic series); validated against the US Naval Observatory phase tables (Δ ≤ 2 min over 25 events, Oct 2026 – Mar 2027; tolerance ±5 min). Illumination uses the Astronomical Almanac's abridged lunar series (≈0.3° longitude accuracy, <0.5% worst-case illumination error). Last verified: 2026-11-16.
A decent pair of 10×50 binoculars
Before any telescope: binoculars reveal the Moon's terminator craters, Jupiter's moons and the Pleiades. A classic, affordable first step for every sky the calculator says is clear.
Check price →FAQ
How accurate is this calculator?
Phase instants come from Meeus's chapter 49 series, accurate to about a minute in the current era. Our implementation matched all 25 published USNO phases from October 2026 to March 2027 within 2 minutes. The abridged lunar series used for illumination adds under half a percent of worst-case uncertainty to the lit fraction.
Why evaluate at 00:00 UT?
A "date" has to map to an instant before any math can run, and UT is the natural neutral choice — the same convention the USNO tables use. Since illumination changes by only a few percent per day, the 00:00 UT snapshot is representative of the whole date for planning; exact event instants are given separately with to-the-minute times.
What about eclipses?
A lunar eclipse can only happen near full moon and a solar eclipse near new moon, but most full and new moons pass without an eclipse because the Moon usually slides above or below Earth's shadow plane. This tool computes phases, not eclipses — check a dedicated eclipse source for those dates.
Related reading
- December 2026 sky guide — full moon calendar table for the month ahead
- What is a blue moon? — when a calendar month squeezes in two full moons
- Full moon effects — what the full moon really does (and doesn't) do